Inverse beta decay

Inverse beta decay, commonly abbreviated to IBD, is a nuclear reaction involving electron antineutrino scattering off a proton, creating a positron and a neutron. This process is commonly used in the detection of electron antineutrinos in neutrino detectors, such as the first detection of antineutrinos in the Cowan–Reines neutrino experiment, or in neutrino experiments such as KamLAND, Borexino, and SAGE. It is an essential process to experiments involving low energy neutrinos (< 60 MeV) such as those studying neutrino oscillation, reactor neutrinos, sterile neutrinos, and geoneutrinos. The IBD reaction can only be used to detect antineutrinos (rather than normal matter neutrinos, such as from the sun) due to lepton conservation.

Reaction

Inverse beta decay proceeds as

${\bar {\nu }}_{e}+p\to e^{+}+n$,

where an electron antineutrino (${\bar {\nu }}_{e}$) interacts with a proton ($p$) to produce a positron ($e^{+}$) and a neutron ($n$). The IBD reaction can only be initiated when the antineutrino possesses at least 1.806 MeV of kinetic energy (called the threshold energy). This threshold energy is due to a difference in mass between the products ($e^{+}$ and $n$) and the reactants (${\bar {\nu }}_{e}$ and $p$) and also slightly due to a relativistic mass effect on the antineutrino. Most of the antineutrino energy is distributed to the positron due to its small mass relative to the neutron. The positron promptly undergoes matter-antimatter annihilation after creation and yields a flash of light with energy calculated as

$E_{\text{vis}}=511{\text{ keV}}+511{\text{ keV}}+(E_{{\bar {\nu }}_{e}}-1806{\text{ keV}})=E_{{\bar {\nu }}_{e}}-782{\text{ keV}}$,

where 511 keV is the electron and positron rest mass, $E_{\text{vis}}$is the visible energy from the reaction, and $E_{{\bar {\nu }}_{e}}$ is the antineutrino kinetic energy. After the prompt positron annihilation, the neutron undergoes neutron capture on an element in the detector, producing a delayed flash of 2.22 MeV if captured on a proton. The timing of the delayed capture is 200–300 microseconds after IBD initiation (≈256 μs in the Borexino detector). The timing and spatial coincidence between the prompt positron annihilation and delayed neutron capture provides a clear IBD signature in neutrino detectors, allowing for discrimination from background. The IBD cross section is dependent on antineutrino energy and capturing element, although is generally on the order of $10^{-44}cm^{2}$ (~ attobarns).

Inverse beta decay may also sometimes refer to the interaction of an electron and proton, creating a neutrino and neutron, although this process is normally referred to as electron capture.

See also

• Kamioka Liquid Scintillator Antineutrino Detector